The plastics manufacturing industry is typically required to compound one to five percent of a pelletized additives package into bulk polymer resin to fabricate plastic parts because of the poor mixing capability of single screw extrusion (SSE) and injection molding (IM) machines. The additives package is a pre-compounded concentrate of functional particles, such as a pigment, and base polymer resin that aids in mixing functional particles within the bulk resin. Pigment additive packages are more commonly known as a color masterbatch. Although the additives package is the minor component, it is typically more costly than the bulk resin.
The base polymer in an additives package is often a low molecular weight polymer with poor mechanical properties. In addition, the base polymer is subject to two shear and heat histories, once during the pre-compounding step, as in extrusion, and secondly during the part fabrication step, as in IM. Subjecting the polymer to multiple processing steps has its disadvantages. For example, every time a polymer is subject to heat and shear forces there is potential for degradation, chemical or otherwise. Multiple processing steps have been found to coarsen the morphology of a previously well mixed system. Mechanical properties are dependent upon morphology; and particles may tend to agglomerate during extrusion. Additionally, multiple processing steps increase manufacturing costs and time.
With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel. The granules are slowly moved forward by a screw-type plunger, i.e., the plastication screw, by which the plastic is forced into a heated chamber, where it is melted. As the plastication screw advances, the melted plastic is forced through a nozzle for delivery to the mold.
Dispersing and distributing pigment, modifiers, filler, particles, reinforcing agents, and other various compounds within a polymer matrix for injection molding are difficult. In most cases, twin screw extrusion (TSE) is commonly used for pre-compounding in order to achieve good mixing. However, single screw extrusion (SSE) offers several advantages, including lower cost, rugged machinery more resistant to abuse, easy and inexpensive part replacement, widely available new or used equipment, easy operation, lower back pressures, and the ability to combine compounding and final product extrusion as a single operation.
Industrial SSE use has lagged because extruders with single screw flights have lacked the multiple elongational flow fields of multi-screw extruders (MSE), which provide simple upstream axial mixing and the ability to degas during mixing. To achieve good dispersion, surface treatments are employed with SSE to promote wetting by the polymer but have not been fully successful nor duplicated the effect of mixing alone achieved with multi-screw extruders. Controlled feeding/melting mechanisms are used with SSE to decrease agglomerate formation and reduce the dispersion necessary for good mixing. To enhance distributive mixing, starve feeding may be used, if the polymer is not subject to degradation. SSE is intrinsically limited in dispersive and distributive mixing but good dispersion can often be achieved by using specialized additives, whereas distributive mixing can equal any MSE compounder with retro-fitted mixing devices. The function of SSE has changed from only plasticating to both plasticating and mixing, achievable by adding a mixing element to the screw.
There are several types of mixing elements suitable for SSE, each with their own advantages and disadvantages. For homogeneity, a combination of both dispersive and distributive mixing is optimal, specifically dispersion followed by distribution. There are no standardized ways to evaluate the compounding ability of a mixer because this will vary with the additives being compounded. For example, it is difficult to quantitatively measure dispersion of filler particles in heavily filled thermoplastics. Comparative studies have been performed in which different types of mixing elements are investigated to improve mixing of hybrid materials systems in SSE. And, there have been attempts to reduce manufacturing costs by improving the compounding role of SSE used in final product manufacture, specifically examining powders in polyolefins and typical liquid additives in various polymers. However SSE is still considered generally unsuitable for dispersive mixing of powders and liquids into polymers, particularly during the plastication step of an injection molding cycle. There remains a need for an SSE capable of achieving distributive mixing of powder and liquid additives in a polymer melt during the plastication step of an injection molding cycle.